Vegan Custard is so simple and one of my favourite things to eat, assuming it comes from childhood when it seemed to be served with every dessert at school and home. It is easy to make and Birds Custard powder is vegan.
I tend to make it using soya milk as I think it naturally makes it creamy or oatly but that tends to be a little more expensive to buy and would not be suitable for a gluten free diet. You could also plant based cream to make it extra rich.
If you are looking to buy ready made custard, Alpro make a soya version in a box in most supermarkets. It is naturally dairy free so lactose free so seems to have it in the free from isles.